The high activity of potassium-containing catalysts for soot oxidation has attracted much attention over the past decade. Unfortunately, potassium containing catalysts usually tend to degrade after repeated thermal cycles due to the loss of potassium. This is oftentimes the case for potassium carbonate (K2CO3) or potassium nitrate (KNO3). This degradation is thought to occur due to loss of potassium through sublimation. Thus the challenge in using potassium and other alkali metal based catalysts is minimizing, if not eliminating, the loss of the potassium or alkali metal.
In many cases, attempts to stabilize potassium have included impregnation of potassium species into a support material. For example, in one attempt to prevent degradation, potassium is incorporated into a more stable crystal structure, such as for example, a perovskite structure. This approach has been common over the past decade, with examples such as potassium catalyzed soot oxidation supported by TiO2, ZrO2, Al2O3, SiO2, etc. Among these support materials, Al2O3 and SiO2 do not show any catalytic effect while TiO2, and ZrO2 show very low activity in soot oxidation. The potassium-TiO2 interaction and the improvement of contact conditions between soot and the catalyst, studied by means of thermal programmed reduction (TPR) and X-ray diffraction (XRD), were offered as an explanation for the activity differences for soot oxidation compared to catalysts supported on alumina. K/ZrO2 with a K/Zr ratio=0.14, synthesized from potassium nitrates was reported as providing the highest activity. The reversible transformation of a bridged NO3− and a monodentate NO3− was suggested as a possible mechanism.
In one example, Ogura et al. (2008) tried to prevent potassium losses by using silica-alumina and zeolites as support materials and impregnating potassium as the active catalytic center. The researchers found that the second test cycle of soot oxidation on K/sodalite showed higher activity than the first run. It appears that the state of potassium was changed under thermal conditions with a minor loss of the crystal structure of sodalite resulting from interaction of Al sites and K in aluminosilicate. However, no extended stability test was performed to reveal the relatively long-term catalytic performance in this study. Relatively extended stability testing was performed by Lopez-Suárez et al. when they studied the effect of copper on potassium stability in K/SrTiO3 catalyzed soot oxidation. According to their 6-cycle repeated TPR results, significant degradation was found between the first and second TPR cycle for all potassium catalysts. Even though the less active Cu/SrTiO3 catalysts do not degrade, the addition of copper into K/SrTiO3 structure does not affect the potassium stability.
Research results show that catalysts with potassium can offer temporary high activity for soot combustion. Nevertheless, such catalysts can be readily degraded due to the loss of active potassium. Conversely, an investigation showed that catalysts with immobile potassium ions that are rigidly bound in the lattice give lower activity and higher soot oxidation temperatures. Neither case is ideally suited for the application on a diesel particulate filter (DPF).
As an alternative one can consider the use of potassium containing glasses as catalysts, as described in U.S. patent application Ser. No. 12/021,108, and incorporated herein by reference in its entirety. This approach relies on the slow passive release of potassium from a glass to provide for renewal of the catalytic surface activity of the glass. The purpose is to mitigate the effect of loss of the active potassium species by providing new ions over time.
In additional, several approaches to modeling glass durability have also been described, primarily for predicting the durability of glasses for nuclear waste storage. In this application, the goal is to minimize leaching from, or corrosion of, the glass, with researchers hoping to predict glass behavior over thousands of years. The approaches usually involve considering a glass as a mixture of species with known free energies of hydration. Glass durability is then predicted based upon thermodynamic hydration equations, with the contribution of the various species scaling with mole percentage of components present in the glass.
To design durable glasses, glass reactant species are selected based on hydration reactions that are expected to occur between the glass and an aqueous solution (acidic or basic). This is based on expectations as to whether cations in the glass will anionically complex with silica or other oxides. This is determined from their relative anionic force, which reflects their relative field strength. Cation species that might be incorporated into a glass can be classified as to whether they are network formers (i.e. ions with high atomic field strengths (F), calculated as the atomic charge (Z) divided by the square of the ionic radius (r)), network modifiers (i.e. ions with low atomic field strengths), or intermediate cations, that can act as either network formers or network modifiers. Network modifier cations are oxide species which are highly anionically associated with [SiO4]−4 tetrahedra. Potassium and sodium are examples of network modifiers that have low field strengths, so are susceptible to leaching.
The field strengths are considered along with the relative partial molar free energies of the hydration reactions (ΔGi) of the cation species that can occur in an aqueous environment. A chemically and electrically balanced hydration reaction can be written for potential components, and the partial molar hydration free energy for each reaction be calculated (ΔGi=ΔG(products)−ΔG(reactants)) for the expected hydration reactions in the environment (e.g. aqueous oxidized basic environment). In another example, Jantzen showed that the thermodynamic free energies of hydration (ΔGhyd) of the silicate and oxide glass components are correlated with their ionic field strengths (F). (Jantzen, C. M. (1992). Thermodynamic Approach to Glass Corrosion. Corrosion of Glass, Ceramics, and Ceramic Superconductors: principles, testing, characterization and applications. D. E. Clark and B. K. Zoitos. Park Ridge, N.J., Noyes Publications: 153-217). The results of these studies provide a starting point for selecting ions for incorporation into a glass with poor weathering characteristics, as is desired for DPF application. Such a glass should incorporate relatively high levels of low field strength, highly negative free energy of hydration ion species (i.e. alkali ions).
If a glass has insufficient durability it will not be well suited for a DPF environment. For example, FIGS. 1A and 1B, respectively, show a simple K2Si2O5 glass before 100 and after 102 multi-cycle soot oxidation. The porous white colored phase in the after photo 102 is a SiO2 rich compound based on XRD and Fourier transform infrared spectroscopy (FTIR) study. This implies there is a decomposition reaction between the K2Si2O5 and C/CO2 during soot oxidation.